LPV521MGX Datasheet, PDF (21/29 Pages) Texas Instruments – LPV521 Nanopower, 1.8V, RRIO, CMOS Input, Operational Amplifier

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1 M:
R_L
LPV521
SNOSB14C – AUGUST 2009 – REVISED FEBRUARY 2013
100 M:
V⁺
-
+
V^-
V_OUT
OXYGEN SENSOR
Figure 65. Precision Oxygen Sensor
INPUT STAGE
The LPV521 has a rail-to-rail input which provides more flexibility for the system designer. Rail-to-rail input is
achieved by using in parallel, one PMOS differential pair and one NMOS differential pair. When the common
mode input voltage (V_CM) is near V+, the NMOS pair is on and the PMOS pair is off. When V_CM is near V−, the
NMOS pair is off and the PMOS pair is on. When V_CM is between V+ and V−, internal logic decides how much
current each differential pair will get. This special logic ensures stable and low distortion amplifier operation
within the entire common mode voltage range.
Because both input stages have their own offset voltage (V_OS) characteristic, the offset voltage of the LPV521
becomes a function of V_CM. V_OS has a crossover point at 1.0V below V+. Refer to the ’V_OS vs. V_CM’ curve in the
Typical Performance Characteristics section. Caution should be taken in situations where the input signal
amplitude is comparable to the V_OS value and/or the design requires high accuracy. In these situations, it is
necessary for the input signal to avoid the crossover point. In addition, parameters such as PSRR and CMRR
which involve the input offset voltage will also be affected by changes in V_CM across the differential pair transition
region.
OUTPUT STAGE
The LPV521 output voltage swings 3 mV from rails at 3.3V supply, which provides the maximum possible
dynamic range at the output. This is particularly important when operating on low supply voltages.
The LPV521 Maximum Output Voltage Swing defines the maximum swing possible under a particular output
load. The LPV521 output swings 50 mV from the rail at 5V supply with an output load of 100 kΩ.
DRIVING CAPACITIVE LOAD
The LPV521 is internally compensated for stable unity gain operation, with a 6.2 kHz typical gain bandwidth.
However, the unity gain follower is the most sensitive configuration to capacitive load. The combination of a
capacitive load placed at the output of an amplifier along with the amplifier’s output impedance creates a phase
lag, which reduces the phase margin of the amplifier. If the phase margin is significantly reduced, the response
will be under damped which causes peaking in the transfer and, when there is too much peaking, the op amp
might start oscillating.
In order to drive heavy capacitive loads, an isolation resistor, R_ISO, should be used, as shown in Figure 66. By
using this isolation resistor, the capacitive load is isolated from the amplifier’s output. The larger the value of
R_ISO, the more stable the amplifier will be. If the value of R_ISO is sufficiently large, the feedback loop will be
stable, independent of the value of C_L. However, larger values of R_ISO result in reduced output swing and
reduced output current drive.
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